Crystals having nonlinear optical (NLO) effect are called nonlinear optical crystals. The said nonlinear optical effects include those of frequency doubling, sum- and difference-frequency generation and parameter amplification. Only noncentrosymmetric crystal could probably possess the nonlinear optical effect. Utilizing the nonlinear optical effects of crystals, it is possible to prepare nonlinear optical devices such as second harmonic generator, up and down frequency converter, optical parametric oscillator and etc. Since the laser frequency could be converted by passing through nonlinear optical devices, many more lasers with useful wavelength could thus be obtained. As a result, much wider applications of the laser could be realized. An all-solid-state blue-green laser system could be realized by utilizing the conversion the frequency of near-infrared laser produced by a solid-state laser apparatus by a nonlinear optical crystal. Such systems have great potential applications in the field of laser technology. Among the nonlinear optical crystals, rare earth frequency-doubling crystals are a unique nonlinear optical material. Since rare earth ions have similar coordination structure and physicochemical properties, the crystal structure and NLO properties do not change dramatically when the rare earth ions are replaced partly or completely by different rare earth ions. Thus improvement of properties of this kind of materials could be easily achieved. Those well-known materials include YAl3(BO3)4 [see former USSR journal Kristall und Technik Vol. 9,63, 1974], NdxY1-xAl3(BO3)4 [see China Physics Letter Vol. 3,413, 1986], Ca4RO(BO3)3 (R=La, Gd, Sm, Er, Y, Lu) [see Chemistry Material Vol. 4,737, 1992] and R2MB10O19 (R represents one or more elements selected from rare earth elements or Y; M selected from Ca, Sr, Ba) [see CN Patent Appl. 1,236,027A and U.S. Pat. No. 6,146,553]. Their second harmonic generation effects are in the range of 2-3 times that of potassium diphosphate. i.e., KDP (KH2PO4). They could be used to prepare nonlinear optical device of wavelength range of blue—green light. Since energy conversion efficiency of laser frequency conversion of nonlinear optical crystal is related to the magnitude of nonlinear optical effect of the crystal, it is necessary to search for crystal materials with much larger nonlinear optical effect in order to raise the output power of the all-solid-state blue—green laser system.